The present invention relates to an LTE compliant RF transceiver. The invention also relates to a modem device comprising such a transceiver.
RF transceivers are known to be a salient component of modem devices in wireless telecommunications devices to provide communication in both directions, i.e. the ability to send and receive at the same time. With the frequency-division duplexing (FDD) transmission technique, transmitter and receiver operate at different carrier frequencies, i.e. uplink (from terminal to base station) and downlink (from base station to terminal) sub-bands are separated by a frequency offset which enables a station to send and receive at the same time. In contrast, time-division duplexing (TDD) is the application of time-division multiplexing to separate transmit and receive signals. It uses the same frequency for uplink and downlink.
3GPP LTE (3rd Generation Partnership Project Long Term Evolution) which is the upcoming standard of 4th generation radio access networks employs both FDD and TDD, and also employs Orthogonal Frequency Division Multiplexing (OFDM) as a downlink modulation scheme. Multiple transmit antennas at the base station side and the mandatory requirement of multiple receive antennas at the mobile terminal side, i.e. user equipment, permit simultaneous transmission of multiple data streams, or data layers, from one base station to one mobile terminal. This transmission method is known as Multiple-Input Multiple-Output (MIMO) OFDM.
Many modern wireless communication systems use both Frequency-Division Duplexing (FDD) and Multiple Input Multiple Output (MIMO). Also, TDD/FDD dual mode devices are known.
According to the LTE standard, the minimum requirements for antennas of a communications terminal is two receive antennas and one transmit antenna, i.e. a typical implementation of an LTE terminal provides a single transmit path and a pair of receive paths. Since in FDD transmit and receive operate at different frequencies, as stated above, two independent PLLs (Phase Locked Loops) are required in the RF transceiver of such a terminal.
FIG. 1 shows a typical known RF transceiver as comprising one transmit (Tx) path and two receive (Rx) paths. Both the Tx side and the Rx side each have a dedicated PLL synthesizer associated therewith for setting the carrier frequency for the respective path. Each path comprises a mixer to receive said carrier frequency from the PLL and use it to convert a respective RF signal into a BB signal or vice versa, and a filter for adapting the signal to the desired bandwidth. In particular, as shown in FIG. 1, a Tx path comprises Tx filter 112 and Tx mixer 114, a first Rx path comprises Rx filter 152 and Rx mixer 154, and a second Rx path comprises Rx filter 162 and Rx mixer 164.
Tx mixer 114 receives a baseband signal (Tx BB in) which has been adjusted in bandwidth by Tx filter 112, and receives a carrier frequency from dedicated Tx PLL 130 to convert the filtered baseband signal up to RF for output (Tx RF out) to a Tx antenna for transmission.
Each Rx mixer receives an RF signal input, Rx RF in 1 and Rx RF in 2, from a separate Rx antenna, and a carrier frequency from a shared Rx PLL synthesizer 140 to downconvert the received RF signal to a baseband signal for supply, via a respective RF filter (152, 162) which adjusts the bandwidth of the signal, to a baseband unit for demodulation.
PLL synthesizers such as Tx PLL 130 and Rx PLL 140 generally comprise a control loop which includes a voltage controlled oscillator (VCO), a reference clock generator, a phase detector charge pump and a loop filter. For more details about the internal structure of a typical PLL used in RF communication devices see e.g. U.S. Pat. No. 7,498,888 (WO2005062471 “Method and arrangement for interference compensation in a voltage-controlled frequency generator”).
There is a continuing demand in mobile communications for increased data rates, speed-up of communication setup and/or communication handling, and power savings.
A general object of the invention is to make the PLL configuration flexible, so that the RF transceiver can support more functionality. A further object of the invention is to provide an improved LTE compliant RF transceiver and an improved LTE compliant modem which allow to speed-up handover procedures in mobile communication.